In contrast to conventional filter-cleaning constructions which worked with mechanical means (vibration) or pressure pulses (with stationary filter and stationary nozzle), and which are unsatisfactory with regard to their degree of cleaning, and particularly in which the mechanical stress on sensitive filters, above all, is too high, filter arrangements of the above type have been developed, in which either the cleaning device, normally in the form of nozzle, is moved and the filter is stationary (U.S. Pat. No. 4,770,118) or a movement of the filter occurs, with the nozzle being stationary. The main purpose of conventional filter-cleaning devices is to clean the filter during operation in such a way that it remains operational, that is to say to regenerate it. In the event of a color change, especially when there are pronounced differences in the color shades, the filters have to be exchanged completely.
Where the nozzles are concerned, there are generally two systems, namely one in which compressed air is blown through the filter from the clean-air side, that is to say from the rear side of the filter remote from the booth interior (German Offenlegungsschrift 3,318,043), and another in which the work is carried out with suction air from the booth interior, that is to say from the "dust" side (German Patent Specification 3,023,044).
Surface filters have the advantage over cartridge filters, which allow a higher degree of separation per se as a result of their larger filter area in relation to the constructional size, in that they can be produced relatively cheaply and permit a very rapid filter exchange in the event of a color change (for example, cling fastenings in needle-felt filters), thereby affording advantages especially when medium or relatively small batch sizes of parts are to be coated. For reasons of effectiveness and, above all, also of cost, different surf ace-filter structures have become known for different intended uses, namely, inter alia, paper, fabric, mineral, metal and also plastic filters.
A fundamental problem where paper filters and sensitive fabric filters are concerned is that there must be no mechanical contact between a cleaning-off nozzle and the filter surface, because otherwise destruction would occur. Consequently, with such filters, the nozzles have to move at a distance from the filter surface. A one hundred per cent cleaning of the filter surface, as required by a color change, therefore cannot be achieved during the passage of a nozzle over it. In addition, as a rule, the mechanical supporting structure for the filter impedes the nozzle cleaning. As a result, whenever a color change, which, as indicated, can become necessary relatively often, is required in the booth, such filters also have to be exchanged. In contrast, with normal fabric filters, contact between the nozzle surface and filter surface is possible, and this, although resulting in higher efficiencies, nevertheless also leads to easier contamination of the filter as result of mechanical clogging/choking. Furthermore, mechanical abrasion causes the formation of fluff, which has to be removed from the cleaned powder material, to be reused if possible, by means of an additional operation.
In order to overcome the disadvantages of these known paper and fabric filters, particularly the need for a filter exchange at each color change, plastic filters are being used to an increasing extent. However, these can be employed, in conjunction with the nozzle systems described, only for a limited time, because, on the one hand, a nozzle guided at a distance from the filter surface does not ensure the necessary sufficient cleaning, whilst a nozzle sliding on the filter surface leads in time to the clogging and especially wear of the plastic surface. These difficulties arise to an increased degree when, for static reasons, plastic filters are designed with a surface grooved on the booth side in order to increase their strength.
Such plastic filters are generally mounted as filter packages and cleaned by means of pressure pulses. Adjacent filter parts are thereby subjected to cleaned-off powder material, and therefore controlled cleaning, such as would be necessary for a color change, is impossible.